EPA-Mandated Filters:

I found this article on "EPA Required Filters Making Fire Trucks Break Down":

The 10News I-Team learned San Diego City Councilwoman Marti Emerald is asking the region's congressional delegation to petition the EPA to change their rules.

The EPA forces every diesel-powered vehicle to have a filter to keep soot out of the air, including fire trucks. The filters usually burn off soot before clogging the engine, but fire trucks don't do enough long-haul driving to get hot enough to burn the soot away.